1. Technical Field
The present invention concerns a circuit for achieving large ignition advance, limitation of the speed of revolutions, and for preventing backfiring in a magnetic ignition system comprising a flywheel and a two-legged iron core.
2. Description of the Related Art
For reasons of safety, the motor in a motor saw or similar device must not rotate in the wrong direction since this means that the saw chain also rotates in the wrong direction, leading to the risk of personal injury. If the fuel/air mixture in the motor is significantly ignited before the piston reaches its turning point when trying to start the motor, the motor can start to rotate backwards, since its kinetic energy is low during start, which means that the piston can be pressed downwards in the wrong direction of rotation. Such a process in a motor is known as xe2x80x9cbackfiringxe2x80x9d. In certain conditions, the motor can also reverse its direction of rotation during operation, which is known as xe2x80x9creverse direction runningxe2x80x9d. The risk for personal injury must be eliminated if the motor starts in the wrong direction.
In order to prevent backfiring, it is desired to ignite the mixture close to the upper turning point of the piston (top dead centre) when running at low speeds of rotation, since the kinetic energy of the motor is then low, this is known as xe2x80x9clow ignition advancexe2x80x9d. On the other hand, at high speed of revolutions, it is desired to ignite the mixture earlier, since this extracts more power from the motor. This is known as using a xe2x80x9chigh ignition advancexe2x80x9d.
An ignition system with a large ignition advance (a large difference between the time of ignition at low speed and the time of ignition at high speed) thus makes it possible to reduce the risk of backfiring at low speed at the same time as making it possible to extract a great deal of power from the motor at high speed. However, this is difficult to achieve with one module that does not have a large ignition advance, since such a module is normally adjusted to an ignition position that is a compromise between starting ignition and ignition for high speed.
The present invention concerns a circuit for achieving large ignition advance, limitation of the rate of revolutions in both directions of rotation, and for preventing backfiring and reverse direction running of a motor.
The circuit on which the invention is based concerns a magnetic ignition circuit that comprises a flywheel and a two-legged iron core. An ignition transformer, a charging coil for generating a charging pulse for a charging circuit and a triggering coil that is connected in antiphase to the charging coil are arranged on the first leg of the iron core in the direction of rotation of the flywheel. The triggering coil generates, to a main switch, a triggering pulse before the charging pulse and a triggering pulse after the charging pulse. Such a magnetic ignition system has a very small ignition advance. The reason for this is that the different coils interfere with each other, which means that time displacements that arise in one coil are coupled to the other coils. This gives rise to the ignition at higher speeds of revolutions (frequencies) tending to occur later, something that in most cases is a disadvantage for the performance of the motor.
The invention, which is intended to solve these problems, is characterised by a time-constant circuit, in this case an RC net, that is connected between the charging circuit and the main switch. Such a net is previously known and is often used as an excess speed protection. According to the present application, however, the time-constant circuit controls a control switch that opens/blocks triggering pulses to the main switch, whereby the net can be used for limiting the speed of revolutions, in order to achieve safety from reverse direction running and to achieve large ignition advance in a cost-efficient manner without the ignition system becoming unnecessarily large and clumsy.
Other characteristics of the invention are specified in the accompanying claims.